Resolution of azeotropic mixtures of chlorosilanes



Patented Aug. 7, 1945 RESOLUTION OF AZE IOTROPIC MIXTURES OF CHLOROSILANES Robert 0. Sauer, Schenectady, N. Y., assignor to General ElectricCompany, a corporation of New York No Drawing. Application May 14, 1942,

Serial N0. 443,024

3 Claims.

The present invention relates to a method of resolving an azeotropicmixture of chlorosilanes. It is specifically concerned with thseparation of a constant boiling mixture of trimethylchlorosilane andtetrachlorosilane (silicon tetrachloride).

In the preparation of methyl silicon chloride by efl'ecting reactionbetween silicon and methyl chloride in accordance with the processesdescribed and claimed in the copending applications of Eugene G. Rochow,Serial No. 412,459, and Eugene G. Rochow and Winton I. Patnode, SerialNo. 412,460, filed September 26, 1941, and assigned to the same assigneeas the present invention, the reaction products comprise a mixture ofchlorosilanes including various methyl chlorosilanes andtetrachlorosilane.

It has been found that these chlorosilane mix tures can not becompletely separated into their various components by fractionaldistillation because the trlmethylchlorosilane and tetrachlorosilanecomponents not only have approximately the same boiling points (about57.5 C.) but also form a constant boiling mixture distilling at atemperature of about 54.5 C. As the trimethyl compound and itsderivatives are particularly useful for many commercial applicationswhere the presence of tetrachlorosilane can not be tolerated, it washighly desirable that some means he found for resolving this azeotropewithout subjecting the compounds to hydrolyzing or resinifyinsconditions.

An obvious method of separation would seem to be the formation of thecorresponding ethers by alcoholysis, using anhydrous alcohols, followedbyiractional distillation of the products. However, this method provedto be either inoperative or commercially impracticable. With the com monalcohols, such as the methyl. ethyl and butyl alcohols, the resultinethers of the trlmethyl silicon derivative formed constant-boilingmixtures with the excess unreacted alcohol. alcohols and dissolvedhydrogen chloride could not be easily removed from these mixtures bywashing with water without considerable hydrolysis of the trimethylsilicon compound. Other common alcohols such as ethyl alcohol could notbe used commercially because the water ordinarily contained thereinquickly hydrolyzed the silicon halides.

I have now discovered that the resolution of the azeotropic mixture ofthe above-mentioned chlorosilanes may be accomplished under anhydrousconditions by treating the mixture with ethylene oxide to convert thecomponents thereof The and fractionating the resulting products. Thetrimethyl silicon derivative, i. e., the trimethyl-2 chloroethoxysilane,can be easily hydrolyzed and may be used in place oftrimethylchlorosilane in any application involving hydrolysis of thissilicon compound. On the other hand, if the chloride is specificallydesired, it may be regenerated by treating the chloroethoxy compoundwith an organic acid chloride in a manner more fully describedhereinafter.

In accordance with my invention, the constant boiling mixture oftrimethyl chlorosilane and tetrachlorosilane, in which the inol fractionof the trimethyl compound is approximately 0.45, is treated withethylene oxide in a manner more fully described in the copendingapplications oi Winton I. Patnode and Robert O. Sauer, Serial Nos.443,022 and 443,023, filed concurrently herewith and assigned to thesame assignee as the present invention. 'I hese applicationsrespectively describe and claim a process of preparing 2-chloroethylsilicate from silicon tetrachloride and new chemical compounds derivedfrom 'organo-chlorosilanesby reaction thereof with reaction between theethylene oxide and the azeotropic mixture of the two chlorosilanescomprises a glass reaction vessel equipped with a reflux condenser(water-cooled) and a gas inlet tube extending to the bottom of thevessel. The azeotropic mixture is placed in this vessel and an excess ofethylene oxide gas is slowly bubbled through the mixture preferably heldat a somewhat elevated temperature, for example -85 C., for about 10hours or until the reaction mass contains substantially no activechlorine. The product is fractionaily distilled, usually at a reducedpressure, to recover the trlmethyl-Z-chloroethoxysllane boiling at about134 C. at 760 mm. (55 56 C./50 mm)... The tetrakis-2-chloroethylsilicate subsequently distills over at a temperature of about 184 C./'?mm. (153-4 C./2 mm.).

If it is desired to regenerate the respective chlorosilanes, this may beaccomplished by treating the chloroethoxy derivative with an organicacid chloride. For example, by heating the trimethyl-2-chloroethoxysilane with an equimolar amount of benzoyl chloride underreflux at a temperature of about C. and in the presence of a suitablecatalyst such as pyridine or a pyridine salt, e. 8., pyridine phosphate,good yields of trimethylchlorosilane may be obtained. 1

- .to the corresponding z-chloroethoxy derivatives, 55 The same degreeof care should be used in stor-- ing the z-chloroethoxysilanes as isused with the chlorosilanes. The chloroethoxy derivatives arehygroscopic and storage thereof for any length of time in contact withair results in a partial or complete hydrolysis thereof.

What I claim as newand desire to secure Letters Patent of the UnitedStates is:

l. The method of resolving an areotropic mixture oftrimethylchlorosilane and tetrachiorosilane which comprises convertingthe chlorosilanes to the corresponding chloroethoxy derivatives byreaction thereof at an elevated temperature with ethylene oxide andseparating the said chloroeth'oxy derivative by fractional distillation.

2'. The method which comprises electing reaction at an elevatedtemneralture between ethylene oxide and a constant boiling mixture oftrimethylchlorosilane and tetrachlorosilane and separating the'chloroethoxy silanes so formed by tractional distillation.

3. The method of resolving an azeotropic mixture oftrimethylchlorosilane and tetrachlorosilane which comprises eiiectingreaction at an elevated temperature between said chlorosilanes andethylene oxide to form trimethyl-z-chloroethoxysilane andtetra-2-chloroethoxysilane, separating said chloroethoxy silanes byfractional distillation, recovering the trimethyl-2-chloroethoxysilane,and eflecting reaction of said trimethyl-2-chloroethoxysilane with anorganic acid chloride at reflux temperatures to formtrimethylchlorosilane.

ROBERT O. BAUER.

